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Instructor’s Manual
GEOL
Chapter 2
Plate Tectonics: A Unifying Theory
Chapter 2
Plate Tectonics: A Unifying Theory
Table of Contents
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Chapter Outline
Learning Outcomes
Chapter Summary
Lecture Suggestions
Enrichment Topics
Common Misconceptions
Consider This
Key Terms
Internet Sites, Videos, Software, and Demonstration Aids
Chapter Outline
Introduction
LO1 Early Ideas about Continental Drift
LO2 What is the Evidence for Continental Drift?
LO3 Earth’s Magnetic Field
LO4 Paleomagnetism and Polar Wandering
LO5 Magnetic Reversals and Seafloor Spreading
LO6 Plate Tectonics: A Unifying Theory
LO7 The Three Types of Plate Boundaries
LO8 Hot Spots and Mantle Plumes
LO9 Plate Movement and Motion
LO10 The Driving Mechanism of Plate Tectonics
♦ Sidebar: Convection in a Pot of Stew
LO11 Plate Tectonics and the Distribution of Natural Resources
LO12 Plate Tectonics and the Distribution of Life
Learning Outcomes
After reading this unit, the students should be able to do the following:
LO1 Review early ideas about continental drift
LO2 Explain the evidence for continental drift
LO3 Describe Earth’s magnetic field
LO4 Explain paleomagnetism and polar wandering
LO5 Explain magnetic reversals and seafloor spreading
LO6 Explain plate tectonic theory
LO7 Identify the three types of plate boundaries
LO8 Describe hot spots and mantle plumes
LO9 Explain plate movement and motion
LO10 Explain the driving mechanism of plate tectonics
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Chapter 2
Plate Tectonics: A Unifying Theory
LO11 Recognize the role of plate tectonics in the distribution of natural resources
LO12 Recognize the role of plate tectonics in the distribution of life
Chapter Summary
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The concept of continental movement is not new. The earliest maps showing the
similarity between the east coast of South America and the west coast of Africa provided
the first evidence that continents may once have been united and subsequently separated
from each other.
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Alfred Wegener is generally credited with developing the hypothesis of continental drift.
He provided abundant geologic, paleontologic, and climatologic evidence to show that
the continents were once united into one supercontinent that he named Pangaea. The
initial reaction of scientists to his ideas was mixed at best.
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The evidence for continental drift includes the fit of the continents, the similarity of rock
sequences and mountain ranges across oceans, the remains of glaciers in locations that
are too warm for glaciers today, and the presence of fossils of the same species on widely
separated continents.
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The hypothesis of continental drift was revised during the 1950s when paleomagnetic
studies of rocks indicated presence of multiple magnetic north poles instead of just one as
there is today. This paradox was resolved by constructing a map in which the continents
could be moved into different positions such that the magnetic data would then be
consistent with a single magnetic north pole.

Seafloor spreading was confirmed by the discovery of magnetic anomalies—reversals of
magnetic polarity—in the ocean crust that were both parallel to and symmetric around the
ocean ridges. The pattern of oceanic magnetic anomalies matched the pattern of magnetic
reversals already known from continental lava flows.
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Plate tectonic theory became widely accepted by the 1970s because the evidence
overwhelmingly supports it and because it provides geologists with a powerful theory for
explaining such phenomena as volcanism, earthquake activity, mountain building, global
climate change, the distribution of the world’s biota, and the distribution of mineral
resources.
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There are three types of plate boundaries: divergent boundaries, where two plates move
away from each other; convergent boundaries, where two plates collide; and transform
boundaries, where two plates slide past each other.

Ancient divergent plate boundaries can be recognized by their associated rock
assemblages and geologic structures. For divergent boundaries, these may include pillow
lavas, rift valleys with thick sedimentary sequences, and numerous dikes and sills.
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Instructor’s Manual
GEOL
Chapter 2
Plate Tectonics: A Unifying Theory
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The average rate of movement and relative motion of the plates can be calculated in
several ways. The results of these different methods all agree and indicate that the plates
move at different average velocities.
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The absolute motion of plates can be determined by the movement of plates over mantle
plumes. A mantle plume is an apparently stationary column of magma that rises to the
surface where it becomes a hot spot and forms a volcano.
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Thermal convection cells within the mantle move the plates. Hot magma rises and forms
new crust; cold crust is subducted back into the mantle at oceanic trenches. Convection
cells may be in just the asthenosphere or in the whole mantle.
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The supercontinent cycle indicates that all or most of Earth’s landmasses form, break up,
and form again in a cycle spanning approximately 500 million years.
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A close relationship exists between the formation of some mineral deposits and
petroleum, and plate boundaries. Furthermore, the formation and distribution of some
natural resources are related to plate movement.
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The relationship between plate tectonic processes and the evolution of life is complex.
The distribution of plants and animals is not random, but is controlled mostly by climate
and geographic barriers, which are controlled to a great extent by the movement of plates.
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GEOL
Chapter 2
Plate Tectonics: A Unifying Theory
Lecture Suggestions
1. Use animations to teach plate tectonics. Technology aids in the understanding of plate
motions with these fantastic animations from the University of California, Santa Barbara.
Global Tectonics topics include Mesozoic Subduction, Pangaea, Himalayan Collision,
Seafloor Spreading, South Atlantic Spreading, and Seafloor Spreading and Magnetic
Reversals. Regional Plate Tectonics and Geologic Histories topics include Pacific
Hemisphere Plate, 80 Ma to Present; N.E. Pacific and W. North American Plate History,
38 Ma to Present; and Plate Tectonic History of Southern California, 20 Ma to Present:
http://emvc.geol.ucsb.edu/downloads.php#IceAge. More plate tectonics animations are
found at the United States Geological Survey website:
http://www.nature.nps.gov/Geology/usgsnps/animate/pltecan.html and at this University
of California Berkeley site http://www.ucmp.berkeley.edu/geology/anim1.html
2. Relative rates of motion between tectonic plates are a difficult concept for many
beginning geology students. The following is an illustration in which members of the
class are designated as specific plates and plate margins that move with a set of tape
measures.
Setting: The western margin of North America 40 Ma, involving the Pacific, Farallon,
and North American plates, a spreading ridge, and a subduction zone. Five students are
designated as follows:
Student 1 = Pacific plate.
Student 2 = West side of spreading ridge; reels out Pacific plate with tape measure.
Student 3 = East side of spreading ridge; reels out Farallon plate with tape measure.
Student 4 = Farallon plate.
Student 5 = North American plate and subduction zone; reels in Farallon plate.
Given: Rate of Pacific plate with respect to North America plate = zero (therefore,
students 1 and 5 are stationary in the illustration). Both students 2 and 3 reel out tape at a
rate of 1 m/10 My (1m = 500km).
Set-up: Two roll-up tape measures are used with a piece of bright electrical tape attached
at each 1 in. interval.
Student 1 stands on one side of the room.
Students 2, 3, and 4 start approximately 3 meters from student 1.
Student 5 stands across the room, approximately 8 meters from student 1.
Students 2 and 3 move together throughout the demonstration and reel out tape at the
same rate.
Student 5 pulls in tape reeled out by student 3.
Student 4 travels along with the tape reeled out by student 3.
The following table is drawn on the blackboard:
RATES OF MOTION
Plate A relative to Plate B rate in km/My
Pacific plate to North America plate. Given as zero.
North America plate to ridge (50).
Pacific plate to ridge (50).
Farallon plate to ridge (50).
Pacific plate to Farallon plate (100).
Farallon plate to North America plate (100).
The illustration is set up and run for each set of plates in the table. The difference in rates
between each pair of students is easily ascertained.
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3. Why was the concept of seafloor spreading necessary for continental drift to be accepted?
How could scientists ignore the overwhelming evidence that the continents could move
over the face of the Earth?
4. Demonstrate the relationship between hot spots and surface volcanic chains with a piece
of paper and a lighted match. As you move the paper over the match, a burn trace is left
with the oldest burn at the farthest distance from the match. Be careful!
5. For a better understanding of seafloor spreading, have the students try this demonstration.
 Have 20 to 30 student volunteers gather in the central aisle of the classroom. They
are referred to as the magma in the chamber underlying the central ridge axis. Have
the students shuffle slowly to the front of the class in pairs, and upon reaching the
front of the room, have each pair separate, one going left and the other to the right.
When they reach the front of the room, have the students hold their arms up if the
instructor's arms are up, or leave them down if the instructor's arms are down.
 In this way, the plates grow at the front of the room, as the students diverge from
each other at the ridge axis and are subsequently replaced by other students emerging
from the magma chamber.
 Students with hands up are normal polarity rocks and hands down are reverse polarity
rocks. The bilateral symmetry of the paleomagnetic pattern and progression of rock
age from oldest (first to come out) to youngest (last to come out) should now be
obvious to most students. Having the students take a small step backwards for each
minute that they are part of the new crust will generate the subsidence that older crust
undergoes. By varying the rate at which the students walk and separate, various
paleomagnetic reversal pattern widths and ridge topographies (East Pacific Rise vs.
Mid Atlantic Ridge) can be generated.
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Chapter 2
Plate Tectonics: A Unifying Theory
6. Note that the direction of plate movement at any given point along a spreading ridge or
subduction zone must be perpendicular to that spreading ridge or subduction zone.
7. Stress the differences between continental and oceanic crust and between the lithosphere
and the asthenosphere. Note especially the rigid behavior, lower density, and brittle,
highly fractured nature of the continental crust.
8. A pot of boiling soup with its surface crust is a useful analogy for describing the process
of convection cell motion.
Enrichment Topics
Topic 1. Pre- and Post-Pangaea Supercontinents. Pangaea, which started splitting up about
200 million years ago, was not the first supercontinent. Rodinia was together about 1 billion
years ago, and there is evidence for a supercontinent as many as 3 billion years ago. How
does the geologic history of Rodinia parallel that of Pangaea? What is the geologic evidence
that the continent was once together and then broke up? The Burke Museum of Natural
History and Culture, University of Washington, 2006:
http://www.washington.edu/burkemuseum/geo_history_wa/Dance%20of%20the%20Giant%
20Continents.htm
Topic 2. Crazy Ideas in Science. Alfred Wegener’s continental drift idea turned out to be
true. Some of the other “Crazy Theories that Turned Out to Be True” involve ice ages and the
asteroid impact theory for the dinosaur extinction. Christian Science Monitor, November 9,
2004: http://www.csmonitor.com/2004/1109/p18s02-hfks.html
Topic 3. Source of Hot Spot Mantle Plumes. Geophysicists and geochemists have argued
about the depth of the sources of mantle plumes. Recent experiments from seismic
tomography show that plumes come from a variety of depths. Well-resolved plumes that
come from the deepest mantle include Ascension, Azores, Canary, Easter, Samoa, and Tahiti.
Hawaii, which is long-lived and voluminous, is less well-resolved and may come from deep.
Evidence for several plumes is only found in the upper mantle, including Bowie, Eastern
Australia, Eifel, Etna, Iceland, Cocos-Keeling, Galápagos, and Juan de Fuca/Cobb. Montelli
et al., Science, 303: 338–343, January 16, 2004.
Topic 4. Do Mantle Plumes Really Exist? This question has been around for a while, and a
good non-technical summary of the controversy is discussed on about.com, which includes
links to other articles: http://geology.about.com/od/platetectonics/a/nohotspots.htm
Topic 5. Is Plate Tectonics Inevitable? Plate tectonics may be a necessary condition for life,
so the existence of plate tectonics on other Earth-like planets is important for determining
whether life may exist on those planets. Using the article “Inevitability of Plate Tectonics on
Super-Earths” in The Astrophysical Journal, have the students discuss why plate tectonics
may be necessary for life to exist on a planet and how likely these large planets are to have
plate tectonics. How can astronomers test whether a large planet is tectonically active and is
home to living things? http://www.iop.org/EJ/abstract/1538-4357/670/1/L45/
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Chapter 2
Plate Tectonics: A Unifying Theory
Common Misconceptions
Misconception: Earth is molten, except for its crust.
Fact: Only small pockets of magma are molten; the rest of the Earth is solid. Some, like the
asthenosphere, are solid but can flow plastically.
Misconception: The crust moves on top of the mantle.
Fact: This is a simplified picture of plate tectonics. In reality, the lithosphere, which includes
both the crust and the solid uppermost mantle, moves about on the underlying asthenosphere,
the part of the mantle exhibiting plastic behavior.
Misconception: Ridges are fixed in space.
Fact: Spreading ridges can move laterally across the upper mantle.
Consider This
1. Why is plate tectonics given the status of theory while continental drift attained only the
status of hypothesis?
2. Since plate movements have been directly measured, is plate tectonics a theory or a fact?
Why?
3. Compare and contrast continental drift and the plate tectonic theory. What does plate
tectonics have that continental drift does not?
4. What is the origin of the term “transform” in transform fault?
5. Why are transform faults associated with spreading ridges, at approximately right angles
to the ridge? Why don’t transform faults intersect subduction zones as well?
6. Why can spreading ridges never be directly connected to subduction zones, but instead
the two must be linked by a transform fault?
Key Terms
continental–continental plate boundary
continental drift
convergent plate boundary
Curie point
divergent plate boundary
Glossopteris flora
hot spot
magnetic anomaly
magnetic field
magnetic reversal
magnetism
oceanic–continental plate boundary
oceanic–oceanic plate boundary
paleomagnetism
Pangaea
plate tectonic theory
seafloor spreading
thermal convection cell
transform fault
transform plate boundary
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Plate Tectonics: A Unifying Theory
Internet Sites, Videos, Software, and Demonstration Aids
Internet Sites
1. Historical Perspective: http://pubs.usgs.gov/gip/dynamic/historical.html
An historical explanation of the development of plate tectonics theory from the United
States Geological Survey.
2. The PLATES Project: http://www.ig.utexas.edu/research/projects/plates/index.htm
From the Institute for Geophysics at the University of Texas, the PLATES project
provides a resource for plate tectonics media, publications, and teaching resources.
3. This Dynamic Earth: http://www.mnh.si.edu/earth/
Four topics are covered in this website by the Smithsonian National Museum of Natural
History, including Plate Tectonics and Volcanoes.
4. Mantle plumes.org: http://www.mantleplumes.org/
A blog discussing the origin of “hotspot” volcanism frequented by some of the top names
in the field.
5. Northern California Earthquake Data Center (NCEDC): http://quake.geo.berkeley.edu/
Information on earthquakes in northern California, including recent epicenters and news.
6. GPS Time Series: http://sideshow.jpl.nasa.gov/mbh/series.html
Global Positioning System (GPS) receivers are used to determine precise geodetic
position measurements each day to chronicle plate movements.
Videos
1. Global Tectonics: Competing Theories. Insight Media, DVD (2006, 22 min.)
The history of plate tectonic theory and how the plates got to be in their current positions.
2. Plate Tectonics. Insight Media, DVD (2004, 25 min.)
The development of plate tectonics theory as an example of the process of scientific
inquiry.
3. World in Motion: Plate Tectonics. Insight Media, DVD (2003, 27 min.)
The supporting evidence for plate tectonics.
4. Making the Pieces Fit: Continental Drift Theory. Insight Media, DVD (2003, 27 min.)
The study of deep earthquakes and their impact on plate tectonics theory.
5. Earth Revealed. Annenberg Media: http://www.learner.org/resources/series78.html
(1992, 30 min., free video):
 #5: The Birth of a Theory. The development of continental drift, seafloor spreading,
and plate tectonics theory.
 #6: Plate Dynamics. The movement and interaction of tectonic plates and the
geological phenomena they account for.
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Plate Tectonics: A Unifying Theory
6. Planet Earth. Annenberg Media: http://www.learner.org/resources/series49.html
(1986, 1 hour, free video)
 #1: The Living Machine: The Theory of Plate Tectonics. Plate tectonics revealed at
geological sites, such as Kilauea volcano and the Atlantic seafloor.
 #5: Gifts from the Earth. Earth’s Natural Resources. Plate tectonics and how it has
revolutionized the way geologists search for natural resources.
Software
1. Plate Tectonics and How the Earth Works. Tasa Graphic Arts, Inc.
The origin and evolution of the continents and the early history of the Earth.
2. The Theory of Plate Tectonics. Tasa Graphic Arts, Inc.
The changes of Earth’s lithospheric plates through time.
Slides
1. Tasa Photo CD-ROM: Tectonics and Mountain Building. Tasa Graphic Arts, Inc.
Earth science photographs to illustrate lectures or Power Point Presentations.
Demonstration Aids
1. A Continental Drift Flip Book. PALEOMAP Project, University of Texas.
2. The Puzzle of the Plates. American Geophysical Union, 18 movable pieces, 4 frame
pieces, 16-page booklet.
3. Continental Drift and Plate Tectonics. Educational Images, Ltd., Slide set.
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